The present invention relates to an image sensor for use in an image input section of facsimile machines, image scanners, etc. More particularly, the invention is directed to an image sensor having a plurality of light-receiving elements arranged in line, each consisting of a photodiode and a blocking diode connected in series such that their poles of the same type are connected to each other.
There have conventionally been proposed image sensors of the type in which each light-receiving element consists of a photodiode and a blocking diode connected in series such that their poles of the same type are connected to each other, and a plurality of such light-receiving elements are arranged in line to constitute a light-receiving element array.
FIG. 5 (plan view) and 6 (sectional view) show an example of a light-receiving element part of such a conventional image sensor. A photodiode PD and a blocking diode BD are formed on a substrate 1 made of, e.g., glass by sequentially depositing and patterning a metal electrode 2 made of, e.g., chromium, a photoconductive layer 4 made of, e.g., amorphous silicon hydride (a-Si:H), a transparent electrode 5 made of , e.g., indium tin oxide (ITO), and an insulating layer 6 made of, e.g., polyimide. Lead lines 7a, 7b made of, e.g., aluminum are connected to the respective transparent electrodes 5 through contact holes 8a, 8b of the insulating layer 6.
The photodiode PD has a light-receiving area A (hatched in FIG. 5) for receiving incident light from above. On the other hand, the blocking diode BD is entirely shielded by the lead line 7b so that no light reaches it.
A plurality (n) of light-receiving elements are arranged in line to form an array. As shown in FIG. 7, the lead lines 7b on the blocking diode BD side are connected to a shift register SR, while the lead lines 7a on the photodiode PD side is grounded through a loading resistor R. An output terminal Tout is connected to the loading resistor R on its photodiode PD side.
The operation of reading out charges in the above conventional image sensor will be described below.
First, the photodiodes PD are scanned by the shift register SR to sequentially receive signals through the respective blocking diodes BD, so that charges are stored in the photodiodes PD.
If light is incident on a certain photodiode PD during one scan, a charge proportional to an amount of the incident light is discharged from that photodiode PD. Then, reset signals (read pulses) are sequentially applied from the shift register SR, so that charges proportional to the released charges are provided to the photodiodes PD, respectively. During this re-charging, a potential generated at the output terminal Tout by a current flowing through the loading resistor R is detected as an image signal (see Japanese Patent Application Unexamined Publication No. Sho. 58-62978).
However, in the above conventional image sensor, when a charge released from the photodiode PD in response to incident light is distributed to the capacitors inherent in the photodiode PD and the blocking diode BD, some current flows through the loading resistor R to compensate for this charge reduction in the photodiode PD. This current impairs stability of the output signal.
To overcome this problem, another type of image sensor as shown in FIG. 8 (plan view) and 9 (sectional view) was proposed. In this image sensor, a plurality of light-receiving elements are arranged in line to form a light-receiving element array, in which each element consists of a first photodiode PD1 and a second photodiode PD2 that are opposed in proximity and connected in series such that their poles of the same type are connected to each other. The first photodiode PDl and the second photodiode PD2 have such a structure that upon illumination they produce the same voltage to prevent a current from flowing outside. That is, they have the same size of the light-receiving area. One of the photodiodes PD1 and PD2 is used as the photodiode PD, and the other as the blocking diode BD. It was expected that the output signal would be stabilized.
However, the above type of image sensor still has the following problem. The aluminum patterns serve to define the light-receiving areas of the photodiode PD and the blocking diode BD, and also serve as the signal read line 7a for the photodiode PD and the bias line 7b for the blocking diode BD. If, as shown in FIG. 10, the aluminum patterns are formed out of position during a photolithography process, the ratio between the light-receiving areas of the photodiode PD and the blocking diode BD is deviated from the design value (e.g., 1:1). As a result, the voltages of the diodes PD and BD when they are illuminated become unequal, that is, some current flows outside. This current impairs stability of the output signal.